1. Field of the Invention
A method for indirect gas species monitoring based on measurements of selected gas species is disclosed. Application of in situ absorption measurements of key combustion species, such as O2, CO, CO2 and H2O, can be used for process control and optimization. In situ absorption measurements offer a number of advantages over conventional extractive sampling in terms of maintenance and response time. However, the gas species accessible by near or mid-IR techniques are limited to species that absorb in this spectral region. Additionally, the absorption strength must be strong enough for the required sensitivity and isolated from neighboring absorption transitions. By coupling the gas measurement with a software sensor gas, species not accessible from the near or mid-IR absorption measurement can be predicted.
2. Description of Prior Art
It has long been recognized that knowledge of the exhaust gas composition from a process can be used in process optimization through control algorithms. Improvements in energy efficiency, pollutant minimization, and production increase and/or quality control are some of the benefits experienced by coupling gas composition measurements to a control system. For example, in the manufacturing of secondary steel, electric arc furnaces are used to melt the scrap metal. These processes operate in a batch mode, by filling the vessel with scrap and then introducing an AC or DC electrode to form a high temperature plasma, as an energy source to melt the scrap. In addition, assist burners are used to transfer additional chemical heat to the process. Quality control of the melt is conducted by extracting molten samples from the bath to test for the desired carbon content.
During this melt process the exhaust gas released from the vessel can contain high concentrations of CO and H2, greater than about forty percent, as a result of incomplete combustion of O2 and fuel in the furnace. Sources of CO and H2 can be traced to scrap mix characteristics that can contain oils, paints, and other hydrocarbon-based material. At different phases of the melt process, the levels of CO and H2 can vary dramatically, due to the random nature of the scrap trapping and to the release of gas in puffs when sections of the melt collapse. The release of these combustible gases can represent more than about thirty percent of the furnace off-gas, resulting in a tremendous loss of chemical energy.
Recovering energy available in the EAF exhaust gas has been demonstrated by conducting post-combustion of the off-gas with O2 injection. Gregory, RESULTS OF ALARC-PCTM POST-COMBUSTION AT CASCADE STEEL ROLLING MILLS, INC., 1995 Electric Arc Furnace Conference Proceedings, p. 211-217 (1995); Jones, et al., POST-COMBUSTION, A PRACTICAL AND TECHNICAL EVALUATION, 1995 Electric Arc Furnace Conference Proceedings, p. 199-210(1995) and Grant, et al., EFFICIENCY OF OXYGEN TECHNOLOGIES IN THE EAF, The AISE Steel Foundation 200 Proceedings, Pittsburgh, Pa. (2000). In these examples, the amount of O2 injected was monitored by knowing the off-gas composition, to prevent overloading or underloading the furnace with O2. Additionally, the off-gas composition was monitored with conventional extractive sampling techniques. This is currently the accepted practice, not only on EAF processes, but also on the majority of combustion processes.
Off-gas analysis from a combustion process can be performed using an extractive sampling probe that is typically water-cooled and inserted into the process. The sequence of events in extractive gas sampling are as follows: 1) a gas sample is pulled through the probe inserted into the process quenching the reaction mixture; 2) passed through a chiller for water removal; 3) passed through a filter for particle removal; 4) compressed by the sampling pump; 5) directed through an analyzer or series of analyzers to measuring dry gas concentration. The analyzer used typically consists of one of the following types: a gas chromatograph, mass spectrometer, nondispersive infrared or dispersive infrared. Gas chromatographs perform a batch analysis, and therefore have the slowest response time from the techniques list. Mass spectrometers provide continuous monitoring with fast-response times, but are sensitive to dirty gases and steams. Moreover, interpretation of the mass spectra is complicated by overlapping mass fragments. This is most evident when interpreting spectra containing CO and N2 species, since both have the same atomic mass unit. For these reasons either dispersive or nondispersive IR analyzers are generally used.
For off-gas analysis of combustion processes, the key species typically monitored are O2, CO, CO2 and H2. Generally, H2O is not monitored, since the volume is high from the combustion process; only the dry gas is analyzed. In the case of EAF off-gas analyses, CO and CO2 are monitored with NDIR instruments, O2 detection is by either resonance paramagnetic or electrolytic cells, and solid-state analyzers are used for H2 detection.
Though extractive sampling has a long history and is an accepted practice for many combustion applications, disadvantages such as slow response time, susceptibility to probe plugging and corrosion, and being a single point measurement, hamper acceptance of this approach as a continuous routine means for process monitoring. In particular, on processes with high levels of particulate matter, such as the EAF, waste incineration, and gasification, maintaining a sampling probe becomes difficult. To avoid these problems, alternatives to extractive sampling are emerging using non-intrusive optical techniques.
A number of in situ demonstration measurements using optical techniques such as diode lasers have been conducted on harsh combustion processes. In this case, a diode laser is tuned at the frequency of an absorption transition of the molecule of interest. A number of examples in the literature demonstrate the use of diode lasers to monitor the exhaust gas from a process. For example, Sandia National Laboratory demonstrated analysis of the off-gas using diode lasers operating in the mid-JR, working with the American Iron and Steel Institute. In their patent WO 99/26058, they disclose using a diode laser propagating through the gap between the furnace exhaust and the main exhaust to measure CO, CO2 and H2O.
Near-IR monitoring of CO and O2 on an EAF has been discussed by Dietrich, et al., LASER ANALYSIS OF CO AND OXYGEN IN EAF OFF-GAS, 59th Electric Furnace Conference and 19th Process Technology Conference Proceedings, Iron and Steel Society (2001). In this case, water-cooled pipes are used on each side of the furnace gap to launch and receive the beam. The optical techniques demonstrated are not only non-intrusive, but also provide real-time information on the gas composition eliminating any issues related to measurement delay times. However, in the case of an EAF process where significant levels of H2 are present, an in situ optical absorption measurement is not feasible, since absorption transitions for H2 occurs in the Lyman bands between 850 and 1108 Å (Okabe, Photo-Chemistry of Small Molecules, John Wiley & Sons, New York, page 166 (1978)), which is in the vacuum ultraviolet spectral region.
Hence, no reliable, accurate and real-time method is currently available for monitoring off-gas species concentrations in high-temperature, high-particulate combustion processes, such as in electric arc furnaces, which has prevented so far the dynamic control of such processes and the optimization of their efficiency.
Thus, a problem associated with methods for sampling and continuously monitoring off-gases from an industrial furnace that precede the present invention is that they provide a slow response time and thereby do not adequately indicate process conditions to enable optimal process control.
A further problem associated with methods for sampling and continuously monitoring off-gases from an industrial furnace that precede the present invention is that they do not provide point measurements, and therefore do not allow sampling the off-gas specifically in the desired locations of the exhaust stream.
Yet another problem associated with methods for monitoring the off-gases of an industrial combustion process that precede the present invention is that they are susceptible to probe plugging and corrosion.
An even further problem associated with methods for monitoring the off-gases of an industrial combustion process that precede the present invention is that they require undue replacement of the monitoring equipment.
Still a further problem associated with methods for monitoring the off-gases of an industrial combustion process that precede the present invention is that they do not provide continuous, near real-time measurements of the species concentration in the waste gases, with acceptable accuracy, so as to facilitate an adapted dynamic monitoring of process characteristics.
In contrast to the foregoing, the present invention provides a method for indirect monitoring that uses tunable diode lasers that seeks to overcome the foregoing problems and provide a more simplistic, more easily constructed and relatively reliable methodology.
For the foregoing reasons, there has been defined a long felt and unsolved need for a method for indirect monitoring that uses tunable diode lasers that seeks to overcome the problems discussed above, while at the same time providing a simple, easily constructed and maintained design that facilitates more reliable process control.